The neurotransmitter of the tract from the entorhinal cortex to the hippocampus (the perforant pathway) is not known, but is thought to be an excitatory amino acid, probably aspartic or glutamic acid. To learn which one is involved, analysis of putative amino acid transmitters (GABA, glycine, aspartate, glutamate and taurine) was made by high pressure liquid chromatography (HPLC) in rat hippocampus after surgical lesion of the entorhinal cortex, or lesion of intrinsic neurons in the hippocampus with kainic acid. Destruction of the perforant pathway caused a decrease in the content of aspartate while the content of the other amino acid was unchanged. The content of all the amino acids measured decreased after injection into the hippocampus of kainic acid. Lesion or the perforant pathway in kainic acid-treated rats caused a further decrease only of the aspartate content. The experiments suggest that aspartate is the neurotransmitter of the perforant pathway.